The passage of large molecules into the endoneurial environment of large peripheral nerves is regulated by two distinct barriers, the perineurial nerve barrier (PNB) and the blood nerve barrier (BNB). The PNB protects the endoneurium from entrance of agents that might be present around the nerve, whereas the BNB regulates the movement of intravascular agents out of endoneurial blood vessels. Anatomically, the PNB and BNB arise respectively because of the formation of tight (impermeable) junctions between the perineurial sheath cells and the endothelial cells of endoneurial blood vessels. In the present experiments, we have examined in rats at the light and electron microscopical level whether a PNB and BNB form in a segment of nerve formed in a rejected nerve or in a silicone tube. In addition, the barriers were studied in cryopreserved (i.e., living not frozen killed) nerve isografts. We also determined whether cryopreservation (in dimethyl sulfoxide and formamide) altered nerve allograft antigenicity in rats. Normally, the barrier tracer protein horseradish peroxidase (HRP, an enzyme of 40,000 M.W.) is kept out of the endoneurium by the tight junctions of the PNB and BNB. However, reaction product for HRP activity was observed in the endoneurium of the nerve segments that reformed after 4-6 months in rejected nerves or in silicone tubes, indicating that these barriers were leaky. In contrast, the barriers were intact in cryopreserved nerve. Our data indicate that barriers do not develop in rejected nerve or in silicone tubes even though these conduits permit axonal regeneration through them. Cryopreserving nerve allografts for 2 hours to 5 weeks did not alter their antigenicity. These grafts were rejected by non- immunosuppressed rats but survived in hosts treated with the immunosuppressive agent cyclosporin. Our results indicate the feasibility of banking nerve allografts, but their survival, after grafting, still immunosuppression.